This invention relates to mobile (or portable) cellular communication systems, and more particularly to an antenna apparatus for use by mobile stations to provide beam forming transmission and reception capabilities.
Widely deployed communication systems, such as cellular mobile telephone systems, provide wireless communications between a base station and one or more mobile stations. The base station is typically a computer controlled set of transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile stations. The base station antenna also receives reverse link radio frequency signals transmitted from each mobile unit. Each mobile station also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse links signals. A typical mobile station is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In certain cellular systems, multiple mobile stations may transmit and receive signals on the same frequency, but in different time slots or with different modulation codes, to distinguish signals sent to or received from individual stations.
Another increasingly popular type of wireless data communication system is the Wireless Local Area Network (WLAN) such as specified by the Institute for Electrical and Electronic Engineers (IEEE) standard 802.11a and its variants 802.11b and 802.11g. Such WLANs are similar to cellular systems in that a coverage area is divided into cells (called Basic Service Sets or BSS in the WLAN nomenclature) that are controlled by a control base station, known as an Access Point. The remote mobile stations, as in a cellular system, then use the base station to access a communication network. WLANs are another type of wireless communication system in which radio frequency channels are shared.
The most common type of antenna for transmitting and receiving signals at a mobile station is a monopole or omnidirectional antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the station. The transceiver receives reverse link signals to be transmitted from circuitry within the station and modulates the signals onto a carrier signal at a specific frequency assigned to that station. The modulated carrier signal is transmitted by the antenna element. Forward link signals received by the antenna element at a specific frequency are demodulated by the transceiver and supplied to processing circuitry within the mobile station.
The signal transmitted from a monopole antenna is omnidirectional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal in one direction versus detection of the same or a different signal coming from another direction. Generally, a monopole antenna does not produce significant radiation in the azimuth direction. The antenna pattern is commonly referred to as a donut shape with the antenna element located at the center of the donut hole.
A second type of antenna that has been used by mobile stations is described in U.S. Pat. No. 5,617,102. The system described therein provides a directional antenna comprising two antenna elements mounted on the outer case of a laptop computer, for example. The system includes a phase shifter attached to each element. The phase shifter may be switched on or off to effect the phase of signals transmitted or received during communications to and from the computer. By switching the phase shifters on and regulating the amount of phase shift imparted to the signals input thereto, the antenna pattern (which applies to both the receive and transmit modes) may be modified to provide a concentrated signal or beam in the selected direction. This is referred to as an increase in antenna gain or directionality. The dual element antenna of the cited patent thereby directs the transmitted signal into predetermined quadrants or directions to allow for changes in orientation of the station relative to the base station, while minimizing signal loss due to the orientation change. In accordance with the antenna reciprocity theorem, the antenna receive characteristics are similarly effected by the use of the phase shifters.
Wireless systems in which the mobile stations must share access to radio channels are recognized as being interference limited systems. That is, as more mobile stations become active in a cell and in adjacent cells, frequency interference becomes greater and thus error rates increase. As error rates increase, to maintain signal and system integrity, the operator must decrease the maximum data rates allowable. Thus, another method by which data rate can be increased is to decrease the number of active mobile stations, thus clearing the airwaves of potential interference. However, this is rarely an effective mechanism to increase data rates due to the lack of priority assignments to the system users.